Field of the Invention
The present invention relates to an analog-to-digital (AD) conversion apparatus, and a solid-state imaging apparatus and an imaging system that include the AD conversion apparatus.
Description of the Related Art
In Japanese Patent Application Laid-Open No. 2002-232291, a two-step AD conversion method of obtaining high-order bits and thereafter obtaining low-order bits is disclosed as a technology for reducing power consumption of an AD conversion apparatus. The AD conversion apparatus disclosed in Japanese Patent Application Laid-Open No. 2002-232291 compares a pixel signal to a staircase reference signal, and measures a count value until an output voltage of a comparator is inverted as a high-order bit. After that, a switch is brought into non-conductive to disconnect the reference signal, and a voltage of the reference signal at this time is held in a first capacitor C1. Next, a voltage obtained by superimposing a reference signal that is small in step width than the above-mentioned reference signal on the held voltage via a second capacitor C2 is input to the comparator. This voltage is compared to the pixel signal, and a count value until the output voltage of the comparator is inverted is measured as a low-order bit. The technology for realizing the two-step AD conversion in this manner is disclosed in Japanese Patent Application Laid-Open No. 2002-232291.
In the AD conversion apparatus disclosed in Japanese Patent Application Laid-Open No. 2002-232291, a signal line for supplying the reference signal for obtaining high-order bits and a signal line for supplying the reference signal for obtaining low-order bits are switched by the switch. When the signal lines are switched by the switch, noise caused by the switch may enter the signal held in the first capacitor C1. The signal held in the first capacitor C1 is used as a comparison signal for obtaining high-order bits, and hence the noise may cause deterioration in conversion accuracy. Thus, the AD conversion apparatus disclosed in Japanese Patent Application Laid-Open No. 2002-232291 may be insufficient in conversion accuracy.